The key word in your title is "decisive". What constitutes a decisive defeat? Ukraine might consider a decisive victory simply reclaiming all its territory. The EU might conder a decisive victory the effective destruction of Russia's ability to wage war for at least 10 years.

Some Russians might consider the war a decisive victory if Putin is driven from office, so the Russians can get back to building their economy and ending the loss of so many young men.

If a decisive victory is Ukraine marching up to the outskirts of Moscow and negotiating an unconditional surrender... probably won't happen. Nor does anyone really want that.

The longer the war continues, the more likely Ukraine will end up with either its borders before this war or its borders before Russia invaded Crimea. That combined with the knowledge that the mighty Russian bear is not nearly as threatening as people thought would be a huge victory for western Europe.

Imagine Russia making threats and NATO yawning. That is a decisive victory.